The present invention relates to an image forming apparatus in which toner images formed on image carrying members through an electrophotographic image forming process are transferred onto such a recording medium as a sheet of printing paper, hereinafter referred to as the printing paper, by means of an endless belt, the image forming apparatus having a full-color mode and a monochrome mode for printing full-color and monochrome images, respectively, as well as a standby mode for performing standby processing operation. More particularly, the invention pertains to a sensor for detecting the currently selected mode of the image forming apparatus.
There is a growing need today for electrophotographic image forming apparatuses featuring not only monochrome but also full-color printing capabilities. Under these circumstances, electrophotographic full-color image forming apparatuses are being developed these days. Typically, a full-color image forming apparatus forms a printed image by using different color toners corresponding to multiple color image data obtained by separating a full-color image. Specifically, the full-color image forming apparatus reads (or scans) an original full-color image by using filters for the three additive primaries (red, green, blue) to obtain data on the original image, produces image data for the three primary colors (cyan, magenta, yellow) and black from the scanned data, develops visual color images using toners of the individual colors (including black) based on the image data, and reproduces the original full-color image by superimposing the developed color images, for example.
It is necessary for this kind of full-color image forming apparatus to perform an exposure process, a development process and an image transfer process for each color as well as alignment (registration) of the individual color images, which is critical. For this reason, the full-color image forming apparatus generally gives an impression that full-color image forming operation is too slow compared to monochrome image forming operation.
One example of a previous approach to the resolution of this problem is disclosed in Japanese Laid-open Patent Application No. H10-039651. This publication proposes a tandem-type full-color image forming apparatus which includes a rotatable endless belt made of semiconductor material, the endless belt having multiple image forming areas arranged in line along a turning direction of the belt on an outer surface thereof. In this full-color image forming apparatus, visual color images of different colors are formed on the individual image forming areas of the endless belt and transferred onto a sheet of printing paper to produce at least one full-color image while the endless belt turns once.
Another example of a conventional tandem-type full-color image forming apparatus employs an intermediate image transfer method, in which visual color images of different colors are formed on cylindrical surfaces of photosensitive drums, each of which serves as an image carrying member having an image forming area, and transferred onto an outer surface of an endless belt (intermediate transfer belt) with the individual visual color images overlaid exactly in register with one another. The color images superimposed on the outer surface of the endless belt are then transferred onto a sheet of printing paper to produce a full-color image.
One known example of the aforementioned tandem-type full-color image forming apparatus employing the intermediate image transfer method has a full-color mode for producing full-color images, a monochrome mode for producing monochrome images and a standby mode for performing standby processing operation.
FIGS. 1A, 1B and 1C are front views of an image transfer unit 200 used in the conventional tandem-type full-color image forming apparatus.
FIG. 1A shows a situation in which the image forming apparatus is in the full-color mode. In the image transfer unit 200 illustrated in FIG. 1A, all of vertically movable image transfer rollers 13A to 13D which are held by roller up/down arms 21A to 21D inside a loop-shaped moving path of an endless belt 11 descend so that an outer surface of the endless belt 11 is brought into contact with cylindrical surfaces of image carrying members (photosensitive drums) 101A to 101D for individual colors, respectively. Visual color images formed on the cylindrical surfaces of the individual image carrying members 101A to 101D based on image information separated into individual color components (including black) are transferred onto the outer surface of the endless belt 11 (primary image transfer) and then onto a sheet of printing paper (secondary image transfer).
FIG. 1B shows a situation in which the image forming apparatus is in the monochrome mode. In the monochrome mode, only the image transfer roller 13A facing the image carrying member 101A on which a visual black image is formed based on the image information descends so that the outer surface of the endless belt 11 is brought into contact with the cylindrical surface of the image carrying member 101A only. Subsequently, the visual black image is transferred from the cylindrical surface of the image carrying member 101A onto the outer surface of the endless belt 11 and then onto a sheet of printing paper.
FIG. 1C shows a situation in which the image forming apparatus is in the standby mode. In this mode, all of the image transfer rollers 13A to 13D ascend and the outer surface of the endless belt 11 comes apart from the cylindrical surfaces of the individual image carrying members 101A to 101D as illustrated.
The image transfer rollers 13A to 13D move up and down as peripheral surfaces of a first rotary cam 23A and a second rotary cam 23B are displaced as a result of rotary motion thereof. More specifically, when the first and second rotary cams 23A, 23B rotate, a sliding member 22A for monochrome image forming and a sliding member 22B for full-color image forming which are in contact with the peripheral surfaces of the rotary cams 23A, 23B move back and forth horizontally, whereby the roller up/down arms 21A to 21D swing and the image transfer rollers 13A to 13D move up and down.
The image forming apparatus further includes a monochrome image forming sensor 30 and a full-color image forming sensor 31 for detecting whether the image forming apparatus is in the full-color mode, the monochrome mode or the standby mode from the locations of the sliding members 22A and 22B. The monochrome image forming sensor 30 includes an optical sensing device 30A for monochrome image forming disposed inside the image forming apparatus and a light shielding member 30B disposed on a rear side of the image transfer unit 200 face to face with the optical sensing device 30A. Similarly, the full-color image forming sensor 31 includes an optical sensing device 31A for full-color image forming disposed inside the image forming apparatus and a light shielding member 31B disposed on the rear side of the image transfer unit 200 face to face with the optical sensing device 31A.
The light shielding member 30B is a generally L-shaped element of which one end is attached to the sliding member 22A. When the sliding member 22A moves back and forth, the light shielding member 30B swings and the other end of the light shielding member 30B interrupts light emitted by the optical sensing device 30A. Likewise, the light shielding member 31B is a generally L-shaped element of which one end is attached to the sliding member 22B. When the sliding member 22B moves back and forth, the light shielding member 31B swings and the other end of the light shielding member 31B interrupts light emitted by the optical sensing device 31A. When a light path of the optical sensing device 30A is blocked by the light shielding member 30B, the optical sensing device 30A outputs an “OFF” signal to a control unit (not shown) of the image forming apparatus. Also, when a light path of the optical sensing device 31A is blocked by the light shielding member 31B, the optical sensing device 31A outputs an “OFF” signal to the control unit. As the optical sensing device 30A and/or the optical sensing device 31A individually outputs the OFF signal in predetermined patterns, the control unit can recognize how the endless belt 11 is shaped (FIG. 1A, 1B or 1C), that is, in which mode the image forming apparatus is currently operated.
When the image transfer unit 200 is taken out of the image forming apparatus, the light shielding members 30B, 31B are also removed from the apparatus. In this state, the control unit can not recognize which mode is currently selected, because signals output from the optical sensing devices 30A, 31A are of the same pattern as in the standby mode. Therefore, the image forming apparatus is provided with a dedicated sensing device for detecting whether the image transfer unit 200 is mounted in position to prevent the image forming apparatus from beginning any image forming operation with the image transfer unit 200 removed therefrom.
In the aforementioned structure of the image forming apparatus employing the optical sensing devices 30A, 31A and the light shielding members 30B, 31B, the provision of the dedicated sensing device for detecting whether the image transfer unit 200 is installed in the image forming apparatus would result in an increase in manufacturing cost.